Modern clocking circuits in integrated circuits require stable frequency references (oscillators) with fast turn-on, small output noise, and small power consumption. Because of stringent frequency stability requirements, an external resonator with a high-quality factor (such as a quartz crystal) is usually employed in conjunction with active components in the integrated circuit for generating the reference oscillations. However, because the number of pins available in an integrated circuit for interfacing with other external circuits is limited, it is advantageous to use only one pin for connecting the resonator. Although only one pin is used for the resonator itself, high-performance oscillator architectures usually employ two capacitors that form part of a positive feedback path and a negative feedback path, respectively, such as in the case of the van den Homberg oscillator. The two capacitances associated with these two feedback paths must be precisely matched for good oscillator performance, which presents unique challenges with respect to testing and calibration. This testing and calibration must occur before the normal start-up of the oscillator.
If a capacitance on the reference/internal side of the oscillator, for example, is larger than the capacitance on the load/crystal/external side, then the oscillator is unstable, in the sense that an oscillation is possible at a frequency much different from the resonator frequency. Conversely, if the reference capacitance is correctly smaller than the load capacitance, which ensures operation at the desired resonant frequency, but the ratio of their values is not sufficiently close to 1, then the oscillator start-up time increases, which is also undesirable. Additional challenges are presented by the unavailability of proper stimuli and/or circuitry for their application in complex systems such as integrated receivers, transmitters, and transceivers, and by the low-supply-voltage environments in which active devices (such as transistors) must operate with small headroom voltages to stay within specification.
Therefore, there is currently a need in the art for properly calibrating the capacitances of an oscillator, such as a single-pin crystal oscillator, while also independently assessing the health of the oscillator's building blocks such as programmable oscillator capacitor blocks and active circuitry.